1. Organic Matter

2. 1.Aluminosilcates are composed of two fundamental units: silica tetrahedra and aluminum octahedra to form sheet-like structures. 2. Cation substitutions can take place in either the tetrahedral sheet or the octahedral sheet which result in negative charge on the mineral. 4. Positively charged cations are attracted to the negative sites on clay minerals which can function as storage for important plant-essential nutrients. 5. Preference for cations at mineral surfaces is dictated by charge, concentration in solution and size of the cation. 3. The total number of negative sites on clay minerals is represented by the Cation Exchange Capacity (CEC).

3. Flocculation means to bring particles together Dispersion pushes particles apart. - charge Al 3+ 2000 B.C. Na +

4. Small, higher-charged cations tend to flocculate clay particles. Large cations with low charge tend to disperse clay particles Ca 2+, Mg 2+, Al 3+ Na +, K +

5. Iron oxides originate from iron-bearing primary or secondary minerals. Reduced iron (Fe 2+ ) occurs in low-oxygen environments and results in a greyish color. Oxidized iron (Fe 3+ ) occurs in high oxygen environments and results in orange-red colors. Water restricts diffusion of oxygen in soils Grey colors interspersed with orange-red colors often indicates the presence of water tables in soils.

6. Iron and Aluminum Oxides Can possess negative, positive, zero charge Potential interaction with cations and anions Cl -, F -, Br -, SO 4 2-, NO 3 -, CO 3 2-, PO 4 3- Anion Exchange

7. Silicate clays possess negative charge due to isomorphous substitution during the formation of the mineral. Iron and aluminum oxides are products of weathering and can possess negative positive or zero charge. Charge derives from Interaction with hydrogen ions in solution. Cation Exchange Cation or Anion Exchange Silicate Clays vs. Al/Fe oxides

8. Soil Organic Matter and Organic Colloids

9. Organic matter plant debris or litter in various stages of decomposition and includes the living organisms in the soil

10. Accumulation of partially disintegrated and decomposed plant and animal residues as well as living biomass. Decomposition principally by soil microorganisms Transitory soil constituent (hours to 100s of years) Requires continual addition to maintain O.M. levels. 1 – 5% (by weight) in a typical, well-drained mineral soil Soil Organic Matter

11. Increases water-holding capacity/porosity Can increase infiltration rates. Aids in soil aggregation, structure Principal source of essential plant nutrients Energy source for soil microorganisms Soil Organic Matter

12. Soil Organic Matter:Natural C-containing organic materials living or dead Microbial Biomass: It is the living population of soil microrganisms. Litter: It comprises the dead plant and animal debris on the soil surface. Macroorganic Matter:Organic fragments from any source which are > 250µm (generally less decomposed than humus). Humus: Material remaining in soils after decompostion of macroorganic matter. Organic Carbon: The carbon content is commonly used to characterize the amount of organic matter in soils. Organic matter = 1.724 x percent organic carbon or, organic matter is 58% organic carbon Categories

13. Composition

14. Plant Materials Carbon (42%) Hydrogen (8%) Oxygen (42%) Nitrogen, Sulfur, Phosphorus, Potassium

15. Composition plants elements compounds

16. Soils and Global Carbon

17. Carbon Soils contain more that 4x more Carbon than all vegetation combined.

18. Amounts 2400 pentagrams (10 15 g) in soil 700 pentagrams as soil carbonates Storage of earth carbon

20. Sugars, starches Crude proteins Hemicellulose Cellulose Fats, waxes Lignins, phenols Compounds Rapid Decomposition Slow Decomposition Decomposition

21. Majority of breakdown results in Carbon dioxide, water, energy and heat Essential elements (N, P, S) are released This is called “mineralization” Highly resistant compounds are formed which remain in the soil for long periods: “humification” Decomposition The biochemical breakdown of mineral and organic materials. Some of the substrate carbon is incorporated into the cells of microorganisms: called “immobilization”

22. Humus Highly resistant to breakdown amorphous, colloidal, organic substances (possessing no plant cellular organization) Can be highly reactive due to carbon content, surface area, and charge

23. Humic Substances a series of high-molecular-weight amorphous compounds Humic Acids Fulvic Acids decay products of higher plants and microbial residue. products of fulvic acids and other decay products

24. Impacts of SOM on Soil Chemical Properties Cation Exchange Soil Acidity Absorption of Organic Compounds

25. Soil Organic Matter, Acidity, and Reactivity

26. Acid Any substance which increases the hydrogen ion concentration in solution. H+H+

27. Common Acids Hydrochloric AcidHCl Sulfuric AcidH 2 SO 4 Nitric AcidHNO 3 Carbonic AcidH 2 CO 3 Acetic AcidHC 2 H 3 O 2 AmmoniumNH 4 +

28. HClH + + Cl - HNO 3 H + + NO 3 - H 2 SO 4 H + + HSO 4 - Strong Acids Reaction goes to completion (complete dissociation)

29. AmmoniumNH 4 + Carbonic AcidH 2 CO 3 Acetic AcidHC 2 H 3 O 2 Weak Acids NH 4 + NH 3 + H + (residual NH 4 + ) H 2 CO 3 HCO 3 - + H + (residual H 2 CO 3 ) HC 2 H 3 O 2 C 2 H 3 O 2 - + H + (residual HC 2 H 3 O 2 ) Incomplete dissociation

30. NH 4 + NH 3 + H + (residual NH 4 + ) H 2 CO 3 HCO 3 - + H + (residual H 2 CO 3 ) HC 2 H 3 O 2 C 2 H 3 O 2 - + H + (residual HC 2 H 3 O 2 ) Incomplete Dissociation NH 4 + NH 3 + H + In pure water, the amount of dissociation is known

31. Incomplete Dissociation NH 4 + NH 3 + H + In pure water, the amount of dissociation is known High amounts of NH 3 and/or H + inhibit dissociation The reaction is inhibited in acid solutions (high (H + ))

32. pH A measure of the amount of Hydrogen ions in water - Log (H + ) Low pH = High amount of Hydrogen ions in water High pH = Low amount of Hydrogen ions in water

33. Low pH = High amount of Hydrogen ions (acidic) High pH = Low amount of Hydrogen ions (basic) Scale: 1 - 14 Battery Acid = < 1 Coca Cola = 2.8 Orange Juice = 4.2 Beer = 4.3 Vinegar = 3.0 Pure Rain = 5.6

34. Incomplete Dissociation NH 4 + NH 3 + H + In pure water, the amount of dissociation is known High amounts of NH 3 and/or H + inhibit dissociation The reaction is inhibited in acid solutions (low pH) Weak Acid

35. Relevance to Soil Organic Matter

36. Organic Matter Carbon (42%) Hydrogen (8%) Oxygen (42%) Nitrogen, Sulfur, Phosphorus Accumulation of partially disintegrated and Decomposed Plant and animal residues.

37. Humus: amorphous, colloidal, organic substances Carbon Oxygen Hydrogen 58% carbon

38. Carbon Hydrogen Oxygen Cation Exchange COOH OH carboxylic Enolic/phenolic Acid functional groups

39. COOH COO - + H + OH O - + H + Both are weak acids (incomplete dissociation) HClH + + Cl - NH 4 + NH 3 + H + (residual NH 4 + ))

40. Low pH = lots of H + = less dissociation = low charge High pH = little H + = more dissociation = high charge COOH COO - + H + OH O - + H + The dissociation of weak acids is inhibited by H + in solution Soil solution

41. Dissociation of Hydrogen COOH COO - + H + OH O - + H + organic strand -C-C-C-C- Soil solution

42. Low pH = lots of H + = less dissociation = low charge High pH = little H + = more dissociation = high charge COOH COO - + H + OH O - + H + Both are weak acids (incomplete dissociation) The dissociation is inhibited by H + in solution

43. Cation Adsorption COOHCOO - + H + COO - + K + COO - --K Adsorbed cation

44. COOH O-O- K+K+ K+K+ COO - OH O-O- O-O- O-O- COO - K+K+ K+K+ Ca 2+ Na + K+K+ Mg 2+ Na + Soil Solution Organic strand Cation Exchange COOH COO- OH O- Functional Groups H+H+ H+H+ H+H+ H+H+

45. - - - - - Na + K+K+ K+K+

46. K+K+ K+K+ K+K+ Mg 2+ Na + Mg 2+ K+K+ K+K+ K+K+ Cations and Organic Matter CEC = 100 – 500 cmol/Kg Kaolinite 2-5 cmol/kg Vermiculite 100-180 cmol/kg

47. Si Al Si Ca 2+, Mg 2+, Zn 2+, Mn 2+, K +, NH 4 +, Na +, H +, Mn 2+ Mineral organic Cation Exchange

48. Si Al Si Total CEC = Mineral Organic + Total Cation Exchange Capacity pH-Dependent

49. Mineral Colloids derive charge from substitution Of lower-charged cations for higher charged cations In the crystal matrix during mineral formation. The Result is permanent negative charge. Organic colloids derive their charge from dissociation of hydrogen ions from acidic functional groups on organic matter/humus. The result is pH-dependent charge Mineral Colloids – 0 – 180 cmol/kg Organic Colloids – 100 – 500 cmol/kg

50. Reactivity of Soil Horizons Contribution to fertility.